What phenotypic classes will be obtained from a cross between AaBb × aabb? In what proportions? What is this cross called?

Parent Genotypes:

  • Parent 1: AaBb
  • Parent 2: aabb

Gametes Produced:

  • AaBb can produce 4 types of gametes: AB, Ab, aB, ab
  • aabb can only produce one type of gamete: ab

Punnett Square and Offspring Genotypes:

We now combine each gamete from AaBb with the only gamete (ab) from aabb:

Phenotypic Classes:

The phenotype of each genotype:
  1. AaBb → Dominant for both traits (A- and B-)
  2. Aabb → Dominant for trait A, recessive for trait B
  3. aaBb → Recessive for trait A, dominant for trait B
  4. aabb → Recessive for both traits

In what proportions of phenotypic ratio?

Phenotypic Ratio (Proportions):

Each combination occurs with equal probability (1 out of 4), so the phenotypic ratio is 1 : 1 : 1 : 1 (Each class appears in 25% of the offspring)

That is:

  • 25% dominant for both traits (AaBb)
  • 25% dominant A, recessive b (Aabb)
  • 25% recessive a, dominant B (aaBb)
  • 25% recessive for both traits (aabb)

What is this cross called?

This is a test cross between a dihybrid individual (AaBb) and a double recessive homozygous individual (aabb). A test cross is done to find out the genotype of an individual showing dominant traits by crossing it with an individual that is recessive for all the concerned traits.






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